1. Field of the Invention
The present invention relates to a method for recovering powdery or granular dry polymer agglomerates from a polymer latex obtained by emulsion or suspension polymerization. More particularly, it pertains to a method to enable drying of the polymer to be done with a small amount of drying energy whereby to be able to recover highly pure powdery or granular polymer agglomerates at low cost by substituting for the water contained in agglomerates of polymer latex particles, organic liquid, or dissolving and extracting the emulsifier or impurities remaining in the agglomerates into the organic liquid.
2. Description of the prior art:
Heretofore, emulsion or suspension polymerization, as a method for obtaining polymers by polymerizing monomers, has found wide industrial uses as an excellent polymerizing method, because of its features including ease of control of the polymerization system, realization of high yield of polymerization, and producing a high degree of polymerization, etc. In these methods, first, the latex obtained by emulsion or suspension polymerization is turned into polymer agglomerates by bringing it into contact with a material (coagulant) having coagulating power like salt or acid, etc. The agglomerates are aggregates of latex particles; if the agglomeration is accomplished at a temperature lower than the polymers' softening points, some gaps will be left between latex particles, producing a porous structure containing water in these interstices. If the agglomeration is made at a temperature higher than their softening points, the latex particles will be fused together simultaneously with their agglomeration, enabling closely packed particles to be formed. Even from porous agglomerated particles which have been agglomerated at a temperature lower than their softening points, similar particles may be obtained, if they are heated above their softening points after agglomerization. Porous particles obtained at temperatures lower than their softening points are so weak as to be liable to disintegration into basic particles of latex, with a result that such operations as dehydration, cleaning, drying, etc., will become very difficult to perform on them for this reason, generally the operations of dehydration, cleaning, drying, etc., are performed after the agglomerated particles have been fused into a slurry by heating them above their softening points. When particles are obtained in this way, materials remaining inside these particles are difficult to get out, as a matter of course, so that high purity polymer powders or granules cannot be recovered, however finely are these particles are comminuted. On one hand, this method is disadvantageous in that a large amount of energy is required for evaporating water when drying after dehydration and cleaning, resulting in high cost. On the other hand, materials retained in the interstices of porous agglomerated particles which have been agglomerated at below their softening points, can be more easily removed if sufficient dehydration and cleaning is possible. However, actually, even after conventional dehydration, a large amount of water is still left, which requires consumption of enormous drying energy.
In such a method, because the polymers are recovered as agglomerates of latex particles and are further thermally fused into powders and granules; the emulsifiers (surfactants) and oil soluble materials used for polymerization and impurities formed as by-products during polymerization are brought into a state of being embedded inside the polymer aggregates, so that their removal is very difficult. Thus the admixture of impurities, including emulsifiers, will induce degradation in quality, as evidenced by coloration, reduction in heat resistance, development of burning, etc., in molded products. This is the largest drawback of this method.
In removing impurities, including the above-mentioned emulsifiers, generally, they are extracted from the recovered polymers after they have been dried and pulverized using an organic solvent. However, even when practicing this method, however finely are the polymers pulverized, only the impurities existing on the surface are usually removed. Too fine pulverization will result in difficulty in recovery after extraction, and so forth. Thus the removal is made only imperfectly and in addition, because the extraction operation is performed after the polymers once have been turned into a dry product, a double operation is required after the drying process, which increases the complexity of the process and increases the cost. Thus this method can not, of necessity, be said to be industrially advantageous.